Most significant terrorist use WMD risk?

In your view, what is the current most significant terrorist – use of WMD risk & why?

(You may include cyber – related activities and synthetic biology advances as possible WMD if you choose.) What separates the highest risk from the others, in your view,? Given your concerns what do you advocate as the elements of a mitigation strategy? Do current efforts include all the elements you would like to see, or are some missing, and if so, what is lacking? (4-6 pages)